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post #1 of 4 (permalink) Old Jun 25th, 03, 03:57 PM Thread Starter
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Location: The Mighty Midwest
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I have NEVER built an engine before, but I have been reading quite a bit about how to do it. What exactly is engine "compression Ratio" (ex. 9.1 to 1) and how would I be able to check for it in a 350 small block 4 bolt main

When you buy an American car, you provide work for a man and his family for three months. -Jay Leno
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post #2 of 4 (permalink) Old Jun 25th, 03, 04:06 PM
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your going to need to understand theres TWO types of compression ratios
the first comonly refered to as the engines compression ratio, is the static compression raio
THE STATIC COMPRESSION RATIO if the DIFFERANCE in VOLUME between the cylinders volume with the piston at BDC (BOTTOM DEAD CENTER) and TDC (TOP DEAD CENTER)
Definition: The Compression Ratio (CR) of an engine is the ratio of the cylinder volume compared to the combustion chamber volume. A cylinder with 10 units of volume (called the sweep volume) and a chamber with a volume of 1 has a 10:1 compression ratio. Static Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder.

DYNAMIC COMPRESSION RATIO
is the volume of the cylinder measured from the point where the valves close, not BDC, the intake valve NORMALLY closes at some point after BDC, and keep in mind the piston compresses NOTHING UNTILL THE VALVES ARE BOTH CLOSED!
PLAY WITH THE CALCULATOR FOR STATIC COMPRESSION BELOW
http://cochise.uia.net/pkelley2/crc.htm
THEN READ THE LINK BELOW AND DOWNLOAD THE DYNAMIC COMPRESSION SOFTWARE
http://cochise.uia.net/pkelley2/DynamicCR.html


things to read


DO YOURSELF A BIG FAVOR
buy these FIVE books,AND CAREFULLY READ THEM... it will be the best money you ever spent, read them, and you will be miles ahead of the average guy. and it will save you thousands of hours and thousands of wasted dollars,

HOW TO BUILD THE SMALL BLOCK CHEVEROLET by LARRY ATHERTON&LARRY SCHREIB
.
HOW TO BUILD MAX PERFORMANCE CHEVY SMALL BLOCKS ON A BUDGET by DAVID VIZARD
.
JOHN LINGENFELTER on modifying small-block chevy engines

how to build & modify CHEVROLET small-block V-8 CAMSHAFTS & VALVTRAINS BY DAVID VIZARD

SMOKEY YUNICK,S POWER SECRETS

THE DISPLACEMENT,CYLINDERHEAD FLOW and cam timing and compression ratio are the major factors in how well the basic combo will work, of these watch the compression ratio matches the cam timing, the larger the displacement the better and cylinderheads are one of the most important parts in the whole combo!



if cams are a mystery please take the time to read these, it will get you a good start, ITS THE CAMS VALVE TIMING THAT HAS THE LARGEST EFFECT ON CHANGING THE STATIC COMPRESSION RATIO TO THE DYNAMIC COMPRESSION RATIO (THE ONLY THING THAT TRUELY MATTERS, BECAUSE ITS THE ONLY COMPRESSION RATIO THE ENGINE EVER SEES)

http://www.newcovenant.com/speedcraf...camshaft/1.htm
(read LESSONs 1-8)

http://www.mercurycapri.com/technica...e/cam/lca.html

http://www.wighat.com/fcr3/confusion.htm

http://www.cranecams.com/instruction...in/camfail.htm

http://www.mercurycapri.com/technica...cam/index.html

http://www.idavette.net/hib/camcon.htm

http://www.cranecams.com/master/adjustvt.htm

http://www.centuryperformance.com/valveadjustment.htm

http://www.totalengineairflow.com/tech/valvelashing.htm

http://www.chevytalk.com/tech/engine/Cam_Selection.html

http://www.chevytalk.com/tech/101/Cam_Theory.html

http://www.babcox.com/editorial/ar/ar119736.htm

http://www.mercurycapri.com/technica...e/cam/vtg.html

http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://home.wxs.nl/~meine119/tech/camqa.html

[ 06-25-2003, 06:45 PM: Message edited by: grumpyvette ]
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post #3 of 4 (permalink) Old Jun 25th, 03, 04:18 PM
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Mark
 
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Posts: 2,372
Angry

It is a fraction that describes the amount of squeeze in an engine.

With the piston all the way down at the bottom, you measure the amount of volume in the cylinder. The piston is down in the bore and it has a "cylinder" of volume above it along with a combustion chamber worth of volume plus the volume created by the gasket thickness and the volume created (or removed) by the valve reliefs/dome etc. The total volume is the numerator (upper number) of the fraction.

The denominator (lower number) of the fraction is that same numerator volume minus the volume (created by the piston movement) that is removed when the piston is at the top of the stroke. You have the combustion chamber volume plus the volume created by the gasket thickness and the valve reliefs/dome etc.

The compression ratio is the number you get when you punch this fraction into your calculator. The higher the compression ratio, the higher the efficiency (POWER) up to the limitations of the fuel octane, all else being equal.

More "squeeze" is better until it starts to detonate. The static C/R is a measurement of squeeze. [img]smile.gif[/img]

-Mark.
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post #4 of 4 (permalink) Old Jun 25th, 03, 05:02 PM Thread Starter
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Thanks!! [img]graemlins/thumbsup.gif[/img]

When you buy an American car, you provide work for a man and his family for three months. -Jay Leno
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